Memcached

memcached is a memory-based object store, developed originally to lighten the load on LiveJournal's database servers.

Some experimental support for memcached is going into MediaWiki. See memcached.doc in the source for more for now.

Broadly speaking, we'd like to be able to dump lots of data in the cache, use it whenever we can, and automatically expire it when changes are made.

Expiration model

explicit expiration times: memcached lets us set an expiration time on an object when we store it. After the time it up, another request for the object will find that it has expired and return nothing to us.
- pro: last-ditch fallback to let data that could be updated badly eventually fall out of the cache
- con: we have to know ahead of time when it will cease to be invalid. hard to do when we're dealing with user edits!

delete cached objects when we know we're doing something that will cause them to be invalid but are not in a position to update them while we're at it
- pro: fairly simple; the item will be reloaded from the database and recached when it's next needed
- con: if this will affect a large number of related items (for instance, creating or deleting a page invalidates the links/brokenlinks tables and rendered HTML cache of pages that link to that page) we may have to hunt them all down and do a lot of updating

include timestamps on cached objects and do our own expiries based on dependencies
- pro: can expire many objects at once by updating a single node they depend on
- con: more things to load; multiple dependencies could be trickier to work with